The present invention pertains generally to ophthalmic laser surgery procedures. More particularly, the present invention pertains to methods for creating corneal flaps for use in corneal reshaping procedures. The present invention is particularly, but not exclusively, useful as a method for using a pulsed laser beam to efficiently create a corneal flap that can be lifted to expose stromal tissue for photoablation.
The cornea provides approximately two thirds of the total focusing power of the eye. Along with the lens, the cornea refracts incoming light and focuses the light on or near the retina. The curvature of the cornea determines where the incoming light will be focused. If the curvature of the cornea is too steep relative to the length of the eye, light from distant sources will be focused in front of the retina, causing a vision impairment known as myopia (near-sightedness). Similarly, if the curvature of the cornea is too flat relative to the length of the eye, light from close sources will be focused behind the retina, causing a vision impairment known as hyperopia (far-sightedness). Finally, when the curvature of the cornea is non-uniform, light from both close and distant sources will fail to properly focus on the retina, resulting in a blurring of vision known as astigmatism.
The refractive errors mentioned above can generally be corrected using eyeglasses or contact lenses. Alternatively, the cornea of the eye can be surgically reshaped to provide the needed optical correction. Currently, the most popular technique for reshaping the cornea is laser-assisted in situ keratomileusis (LASIK). In the widely used LASIK procedure, a microkeratome is used to cut a flap in the cornea. Next, the flap is lifted to expose a bed of stromal tissue. Once exposed, the bed of stromal tissue is vaporized to a prescribed depth using an excimer laser. After laser treatment, the flap is repositioned and allowed to heal. The result is a reshaped cornea. Unfortunately, the creation of a flap using a microkeratome can result in some complications. For example, the effective creation of the flap with the microkeratome often relies on the skill of the surgeon. Complications can result if the flap is cut improperly or completely severed from the cornea. Further, use of the microkeratome requires the eye to be restrained from movement, often causing patient discomfort. Additional drawbacks associated with using a microkeratome to create a flap include the inability to control the shape of the flap and the fact that a relatively large amount of corneal tissue needs to be cut to create the flap.
As an example of another corneal reshaping procedure, U.S. Pat. No. 4,907,586, which issued to Bille et al. for an invention entitled xe2x80x9cMethod for Reshaping the Eye,xe2x80x9d discloses an intrastromal photoablation technique for reshaping the cornea. Importantly for the purposes of the present invention, the above cited Bille patent discloses the use of a pulsed laser beam for photoablation of intrastromal tissue. Unlike the excimer laser, the pulsed laser beam, as disclosed by Bille, penetrates corneal tissue and can be focused at a point below the surface of the cornea to photoablate stromal tissue at the focal point. The ability to reach a subsurface location without necessarily providing a physical pathway allows for volumes of stromal tissue having complex shapes to be accurately disrupted, while minimizing the amount of total tissue disrupted. The present invention uses subsurface photoablation to create a portion of a corneal flap.
When considering the use of subsurface photoablation to create a flap for corneal reshaping, a general knowledge of the anatomy of the cornea of an eye is helpful. In detail, the cornea comprises various layers of tissue which are structurally distinct. In order, going in a posterior direction from outside the eye toward the inside of the eye, the various layers in a cornea are: an epithelial layer, Bowman""s membrane, the stroma, Decemet""s membrane, and an endothelial layer. Of these various structures, the stroma is the most extensive and is generally around four hundred microns thick. Additionally, the healing response of the stromal tissue is generally quicker than the other corneal layers. For these reasons, stromal tissue is generally selected for removal in refractive correction procedures.
In detail, the stroma of the eye is comprised of around two hundred identifiable and distinguishable layers of lamellae. Each of these layers of lamellae in the stroma is generally dome-shaped, like the cornea itself, and they each extend across a circular area having a diameter of approximately nine millimeters. Unlike the layer that a particular lamella is in, each lamella in the layer extends through a shorter distance of only about one tenth of a millimeter (0.1 mm) to one and one half millimeters (1.5 mm). Thus, each layer includes several lamellae. Importantly, each lamella includes many fibrils which, within the lamella, are substantially parallel to each other. The fibrils in one lamella, however, are not generally parallel to the fibrils in other lamellae. This is so between lamellae in the same layer, as well as between lamellae in different layers. Finally, it is to be noted that, in a direction perpendicular to the layer, each individual lamella is only about two microns thick.
Somewhat related to the present invention, a method for finding an interface between layers of lamellae for photoablation using a wavefront analyzer and an ellipsometer was disclosed in co-pending U.S. patent application Ser. No. 09/783,665, filed on Feb. 14, 2001 by Bille and entitled xe2x80x9cA Method for Separating Lamellae.xe2x80x9d As such, the contents of co-pending application Ser. No. 09/783,665 are hereby incorporated herein by reference. In co-pending application Ser. No. 09/783,665, a procedure for creating a corneal flap for a LASIK type procedure was presented. Unlike the present invention, the method disclosed in Bille ""665 involved using subsurface photoablation to cut the entire inner surface for the flap. The present invention, in contrast, contemplates using subsurface photoablation along an interface solely for the purpose of establishing a periphery for the flap. This periphery, in turn, can be used as a starting point to allow layers of lamellae to be separated from each other along an interface by simply peeling the flap away from the remainder of the cornea.
Within the general structure described above, there are at least three important factors concerning the stroma that are of interest insofar as the creation of a corneal flap is concerned. The first of these factors is structural, and it is of interest here because there is a significant anisotropy in the stroma. Specifically, the strength of tissue within a lamella is approximately fifty times the strength that is provided by the adhesive tissue that holds the layers of lamella together. Thus, much less energy is required to separate one layer of lamella from another layer (i.e. peel them apart), than would be required to cut through a lamella. The second factor is somewhat related to the first, and involves the stromal tissue response to photoablation. Specifically, for a given energy level in a photoablative laser beam, the bubble that is created by photoablation in the stronger lamella tissue will be noticeably smaller than a bubble created between layers of lamellae. The third factor is optical, and it is of interest here because there is a change in the refractive index of the stroma between successive layers of lamellae. This is due to differences in the orientations of fibrils in the respective lamella. When consideration is given to using a laser beam for the purpose of creating a corneal flap in a LASIK procedure, these factors can be significant.
In light of the above, it is an object of the present invention to provide an efficient surgical method for creating a corneal flap suitable for use in a corneal reshaping procedure. Another object of the present invention is to provide a method for creating a corneal flap that minimizes the amount of corneal tissue that must be cut to create the flap. It is yet another object of the present invention to provide a surgical method for creating a corneal flap that allows for the accurate positioning of the corneal flap at a predetermined location on the cornea. It is still another object of the present invention to provide a surgical method for creating a corneal flap that allows for the size and shape of the corneal flap to be closely controlled. Still another object of the present invention is to provide a method for creating a corneal flap that is easy to perform and is comparatively cost effective.
In accordance with the present invention, a method for creating a corneal flap suitable for use in a corneal reshaping procedure includes the step of focusing a laser beam to a location between layers of stromal lamellae and photoablating tissue at the interface between these layers. Next, while maintaining the focal point at locations between layers of stromal lamellae, the focal point is moved along a path within the stroma to photoablate a periphery for the flap. With the periphery of the flap established, the edge of the flap is created by making an incision into the cornea that extends from the anterior surface of the cornea to the periphery of the flap.
Once the edge of the flap is created, corneal tissue bounded by the incision can be lifted to mechanically separate the flap from the underlying tissue of the cornea. Specifically, as the corneal tissue bounded by the incision is peeled from the remainder of the cornea, layers of lamellae are mechanically separated from each other to create the flap. More specifically, the layers of lamellae are mechanically separated from each other along the interface between the layers. With the flap created and lifted, an excimer laser can then be used to photoablate exposed stromal tissue and reshape the cornea. After photoablation of the exposed stromal tissue, the flap can be repositioned over the exposed stromal tissue and allowed to heal. The result is a reshaped cornea.
As indicated above, to create the periphery of the flap in accordance with the present invention, the rays of a laser beam must be focused to a location between layers of lamellae to photoablate tissue at the interface between these layers. To position the focal point on the interface between layers, the laser beam is first focused to a start point in the stroma. Preferably, this start point will be at a predetermined distance into the stroma from the anterior surface of the cornea. This predetermined distance will correspond roughly to the desired thickness for the flap (for example, a distance of approximately one hundred and eighty microns can be used).
With the laser beam focused at the start point, tissue at the start point is photoablated by the laser beam to generate a photoablative response (i.e. a bubble is created). The size of this bubble is then measured and compared with a reference value to determine whether the bubble was created on an interface between layers of lamellae or inside a lamella. The measurement of the bubble is preferably accomplished with a wavefront detector. If it is determined that the initial bubble was created inside a lamella, a subsequent bubble is created at a different point in the stroma. In most cases, this subsequent bubble is created at a shorter depth from the anterior surface of the cornea than the initial bubble. The new bubble is then compared to the reference value to determine whether the new bubble was created on an interface between layers of lamellae. This process is continued until a bubble is created having a bubble size indicating that photoablation is occurring on an interface between layers of lamellae.
For the purposes of the present invention, the reference value is chosen to correspond to a hypothetical gas bubble in the stroma that, as a result of photoablation, would have a diameter of approximately fifteen microns. A condition wherein the measured bubble is greater than the reference value is indicative that the photoablation of issue is occurring in the weaker tissue that is located on an interface between layers of lamellae rather than inside of a lamella.
Once a bubble is created indicating that photoablation has occurred at a location on an interface between layers of stromal lamellae, the focal point of the laser is moved along a path within the stroma to photoablate the periphery of the flap. As the laser is moved along the path, the focal point is maintained on the interface between layers of stromal lamellae. From the first point found on the interface, the next point selected for photoablation along the path is chosen at approximately the same depth as the first point. After the photoablation of each point, the resulting bubble is measured and compared to the reference to ensure that photoablation is occurring on the interface. In this manner, photoablation along the path is continued at a constant depth until the measured bubble is less than the reference value. When a bubble is measured to be less than the reference value, the indication is that the focal point is no longer positioned on the interface. When the focal point is no longer positioned on the interface, the depth of the focal point is altered until a bubble is produced that is larger than the reference value (indicating that photoablation is again occurring on the interface).
The process described above is continued until the periphery of the flap is completed. The resulting periphery consists of a cut along an interface between layers of stromal lamellae. Generally, the periphery follows a curved line that is centered approximately on the optical axis of the eye and extends through an arc of about two hundred and seventy degrees. Typically, the entire periphery can be created on a single common interface between layers of lamellae. For this purpose, an ellipsometer is provided to detect a birefringent condition at each location that is photoablated. Specifically, this birefringent condition results from the orientation of fibrils in the lamella. Further, it is known that from one interface between layers of lamellae to another there will be a birefringent change that is manifested as a change in phase of about one half degree. In accordance with the present invention, the detection of the birefringent change can indicate a change from one interface to another. Consequently, detection of the birefringent change can be used to establish and maintain the focal point on a single interface between layers of lamellae while the focal point is moved along the path to cut the periphery for the flap. The result is a periphery for the flap that is created on a single interface between layers of lamellae.
In some cases, due to the anatomy of the cornea or the shape of the desired flap, the entire periphery cannot be created on a single interface between layers of lamellae. In these cases, two or more interfaces may need to be photoablated to create the periphery of the flap. When this is required, it may be advantageous to alter the energy level of the laser beam when transitioning from one interface to another. Specifically, a higher energy is generally required to efficiently photoablate within a layer of lamellae than is required to efficiently photoablate on an interface between layers of lamellae. For example, an energy of approximately five microjoules for a ten micron diameter spot size is suitable for photoablation on an interface between layers of lamellae, while a somewhat larger energy is more efficient for photoablation within a layer of lamellae.